This invention relates in general to ring laser gyroscopes and more particularly to means in such a gyroscope for compensating the effects of dither on an output interference pattern produced by a pair of oppositely traveling laser beams.
Many types of ring laser gyroscopes have been developed. Typical is the apparatus disclosed in U.S. Pat. No. 3,373,650 describing a ring laser gyroscope which employs monochromatic beams of light directed by mirrors in two opposite directions in an optical cavity around a closed loop path about an axis of rotation. Turning the apparatus about the axis of rotation causes the effective path length for each beam to change and results in oscillation at different frequencies in the beams since the frequency of oscillation of a laser is dependent upon the length of the lasing path. The two waves may be combined to generate interference patterns from which a measure of the rotational rate about the axis can be obtained. As is explained in that patent, the difference in frequency between the two beams at low rotational rates is small and they tend to resonate together, or to "lock in", and to oscillate at only one frequency. Therefore, low rotation rates cannot be detected. In U.S. Pat. No. 3,373,650, a solution to this problem is provided in which the apparatus is oscillated or "dithered" to avoid lock-in of the two beams. Another structure of this kind is disclosed in U.S. Pat. No. 3,467,472 and a detailed explanation of the problem and of various proposed solutions is contained in U.S. Pat. No. 3,879,103.
A problem with the use of dither is the introduction into the gyroscope output signal of an extraneous oscillating component. The elimination of this dither induced component is accomplished in many gyroscope designs by means of retroreflectors mounted on the instrument casing or on arms extending from the center of the gyroscope block to an apex thereof. These devices are sensitive to temperature changes and mechanical distortion in the casing or arm, causing errors in the output of the gyroscope. It has also been proposed, for example, in U.S. Pat. No. 3,373,650, to use transparent wedges to compensate the effect of dither on the interference pattern produced by the counter-rotating laser beams. An object of the present invention is to provide a ring laser gyroscope with an improved means of eliminating the dither induced signal component, which is less sensitive to temperature and mechanical distortions.